Recently, a digital projection and display system is commercially available from Texas Instruments Incorporated (TI) under the trademark Digital Light Processing® (also referred as DLP). The DLP digital projection and display system utilizes a digital micromirror device (DMD) in the light path. The digital micromirror device (DMD) is fabricated according to a MEMS (Micro-Electro-Mechanical System) technology. Typically, the DMD includes a great amount of tiny digital light switches. The digital light switches are very small-sized micromirrors allowing for fast and precise rotation in receipt of electronic control signals. Each of the micromirrors is a light switch that is capable of reflecting light in one of two directions. When the micromirror is in an “on” state, the incident light is reflected into the pupil of the projection lens and thus the micromirror appears bright in this “on” state. Whereas, when the micromirror is in an “off” state, the incident light is reflected out of the pupil of the projection lens and thus the micromirror appears dark in this “off” state.
Desired gray scales are achieved by using binary pulse width modulation to control the micromirrors. For example, a digital data of an 8-bit binary code may be converted into a pulse width of 256 discrete levels such that there are 256 bright-to-dark duration ratios for the micromirror. That is, this gray scale sets an image resolution of 256.
FIG. 1 is a simplified diagram illustrating a conventional DMD-type digital projection and display system. In the digital projection and display system of FIG. 1, a light beam emitted by a light source 10 is focused and shaped by a lens assembly 11 and then transmitted to a digital micromirror device (DMD) 12. The light beam is then selectively reflected by the digital micromirror device 12 to create a visual image on a screen 15 through a projection lens set 14. In addition, a color wheel 13 disposed between the light source 10 and the digital micromirror device 12 is a component for displaying colorful images through the digital micromirror device 12. As the color wheel 13 rotates for a revolution in a unit time period, the light beam emitted by a light source 10 successively passes through different sections of the color wheel 13 such that different colors of light are projected onto the digital micromirror device 12 in different time segments. For example, if the color wheel 13 has four light-transmissive sections with different colors (e.g. white, red, green, and blue sections), white, red, green, and blue light beams are successively filtered by the color wheel 13 in different time segments of the unit time period and projected onto the digital micromirror device 12. By using the above stated binary pulse width modulation, different colors with different intensities may be adjusted in each time segment so as to mix a desired color.
Nowadays, with the maturity of the light emitting diode (LED) technology, the conventional white light source is gradually replaced by the LED. That is, the LED may be used as the light source of a projection apparatus.
FIG. 2 is a schematic view illustrating a projection apparatus using light emitting diodes as light sources according to prior art. As shown in FIG. 2, the light source 10 includes a red color light source R, a green light source G, and a blue light source B for emitting a red light beam, a green light beam, and a blue light beam respectively. By means of a reflective mirror 21 and dichroic mirrors 22 and 23, the red light beam is reflected by the reflective mirror 21. The dichroic mirrors 22 permits the red light beam to pass through the dichroic mirror 22 and reflects the green light beam. The dichroic mirror 23 permits the red light beam and the green light beam to pass through the dichroic mirror 23 and reflects the blue light beam. After processed by the reflective mirror 21 and the dichroic mirrors 22 and 23, the red, green, and blue light beams are mixed and then focused by a condenser lens 24. The focused red, green, and blue light beams are then subject to homogenization by an integrator rod 25. The red, green, and blue light beams are then processed by the digital micromirror device 26 to create a visual image on a screen 28 through a projection lens set 27. Since the LED has a quick response speed, the on/off states of the LED may be controlled by a switching circuit. In other words, the use of the switching circuit may replace the color wheel 13 as shown in FIG. 1. Due to the arrangement restriction of the dichroic mirrors 22 and 23, the LEDs of the conventional projection apparatus may emit color beams of respective wavelengths. Under this circumstance, the thermal load of each LED is very heavy. Generally, as the temperature of the LED is increased, the illuminating efficiency of the LED is reduced and thus the overall brightness value of the projection apparatus fails to be effectively enhanced.
Therefore, there is a need of providing a light source module to obviate the drawbacks encountered from the prior art.